The vehicle wheel bearing apparatus includes various types in accordance with applications for a driven wheel as well as a driving wheel. For example, a conventional bearing apparatus for a driving wheel is shown in FIG. 6. The bearing apparatus has an inner member 50 including a wheel hub 51 and an inner ring 52; double row rolling elements 53 and 54; an outer member 55; and a constant velocity universal joint 56 to transmit an engine output to the wheel hub 51. In a driving wheel bearing apparatus, the wheel hub 51 for supporting a wheel (not shown) and a brake rotor 57 is made of medium carbon steel for machine structures, such as S53C, in view of its ease of forging, cutting and heat treatment, and economy in production. The wheel hub 51 and a wheel mounting flange 58 have been ribbed and thinned to reduce their size and weight to improve fuel consumption as well as driving stability. However since the mechanical strength of such a miniaturized wheel hub 51 itself nears the fatigue limit of the medium carbon steel forming the wheel hub 51, it is difficult to further proceed with the miniaturization and reduction of weight of the wheel hub 51.
In the wheel hub 51 where the wheel mounting flange 58 is thinned for weigh reduction purposes, a special countermeasure is required to deal with the concentration of rotary bending stress at the base portion of the outboard side, namely, at a corner 61 extending from a brake rotor mounting surface 59 toward a cylindrical pilot portion 60. Although it is possible to reduce the generated stress by enlarging the dimension of the corner 61, i.e. the radius of curvature, it is also limited by interference due to the brake rotor 57 being mounted on the wheel mounting flange 58.
The applicant of the present application has proposed a vehicle wheel bearing apparatus which increases the strength of the wheel hub 51 as well as reducing its weight without changing the configuration and dimension of the wheel mounting flange 58. In this bearing apparatus, as shown in FIG. 5, the corner 61 of the flange 58 of the wheel hub 51 is formed with a surface hardened layer 62. The surface hardened layer is formed by high frequency induction hardening. This strengthens the corner 61 of the flange 58 where the rotary bending strength is most weakened and thus increases the durability of the wheel hub 51.
Portions other than the corner 61, e.g. a seal land portion which slidably contacts a seal lip is fitted into the outboard side end of the outer member 55, as well as, portions “a”˜“d” from the raceway surfaces to the axially extending stepped cylindrical portion of the wheel hub 51, are also formed with a surface hardened layer 63 by induction hardening. In addition a serrated portion 64 formed within the inner peripheral surface of the wheel hub 51 is formed with a surface hardened layer 65. Thus the rotary bending strength, wear resistance and rolling fatigue life required for these portions “a”˜“d” can be improved (see Japanese Laid-open Patent Publication No. 87008/2002).
Although it is possible, according to the vehicle wheel bearing apparatus of the prior art, to increase the strength of the wheel hub 51 as well as to reduce its weight without changing the configuration and dimension of the wheel mounting flange 58 by forming the surface hardened layer 62 at the corner 61 of the flange 58 of the wheel hub 51, new problems are caused such that the wheel mounting flange 58 is deformed by the heat treatment in the high frequency induction hardening step. Thus, this causes a large surface run-out of the brake rotor mounting surface 59. The problem of surface run-out is also caused by the thinning of the wheel mounting flange 58. The surface run-out of the brake rotor mounting surface 59 gives influence to the run-out of the brake rotor 57. Thus, this causes a brake judder which impairs the driving stability and the driving feeling. In this case, although it is also conceivable to further cut the brake rotor mounting surface 59 by lathe cutting after heat treatment of the wheel hub 51 to modify the deformed portion so as to improve the surface run-out, there also remains the dilemma that a slight step difference is caused between the surface hardened layer 62 of the corner 61 and the unhardened brake rotor mounting surface 59 due to a difference in hardness between the two.